Photoinduced Charge Generation in a Molecular Bulk Heterojunction

Nov 7, 2012 - ABSTRACT: Understanding the charge generation dynamics in organic photovoltaic bulk heterojunction (BHJ) blends is important for providi...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/JACS

Photoinduced Charge Generation in a Molecular Bulk Heterojunction Material Loren G. Kaake,*,† Jacek J. Jasieniak,† Ronald C. Bakus, II,∥ Gregory C. Welch,⊥ Daniel Moses,† Guillermo C. Bazan,†,§,∥ and Alan J. Heeger†,‡,§ †

Center for Polymers and Organic Solids, ‡Department of Physics, §Materials Department, and ∥Chemistry Department, University of CaliforniaSanta Barbara, Santa Barbara, California 93106, United States ⊥ Department of Chemistry, Dalhousie University, Halifax, NS, Canada ABSTRACT: Understanding the charge generation dynamics in organic photovoltaic bulk heterojunction (BHJ) blends is important for providing the necessary guidelines to improve overall device efficiency. Despite more than 15 years of experimental and theoretical studies, a universal picture describing the generation and recombination processes operating in organic photovoltaic devices is still being forged. We report here the results of ultrafast transient absorption spectroscopy measurements of charge photogeneration and recombination processes in a high-performing solution-processed molecular BHJ. For comparison, we also studied a high-performing polymer-based BHJ material. We find that the majority of charge carriers in both systems are generated on 30 nm) and is comparable with the phase-separated domain size. In addition, exciton diffusion to charge-separating heterojunctions is observed at longer times (1−500 ps). Finally, charge generation in pure films of the solution processed molecule was studied. Polarization anisotropy measurements clearly demonstrate that the optical properties are dominated by molecular (Frenkel) exictons and delocalized charges are promptly produced (t < 100 fs).

■

INTRODUCTION Steady advances in the efficiencies of organic solar cells1−5 have been accompanied by a more detailed understanding of the mechanism by which mobile charges are generated.6−8 The active region in a solution-processed organic solar cell is a mixed film of electron-donating and electron-accepting materials known as a bulk heterojunction (BHJ). In earlier work, the electron donor has been a semiconducting polymer, but recent awareness of challenges imposed by purity and batch-to-batch variation has led to the development of solutionprocessed electron-donating molecules.9−11 When blended with fullerenes, the most common electron acceptor, these systems show promising performance.4,12 From a fundamental point of view, the development of any new photovoltaic material begs the question regarding the relevance of preexisting models in describing the photophysics of charge generation. The currently accepted model of carrier photogeneration describes the process as beginning with the excitation of a localized and bound electron−hole pair called a Frenkel exciton (an intramolecular excitation). The exciton moves diffusively until it encounters a donor−acceptor interface, where charge transfer is energetically favorable. The result of the charge transfer process has been described as a bound interfacial pair called a charge transfer (CT) exciton, which separates into mobile charge carriers via a mechanism that has yet to be definitively identified.13−16 In contrast to this exciton diffusion picture, ultrafast generation of mobile carriers (t < 100 fs) is © XXXX American Chemical Society

observed and, in fact, accounts for a large percentage of the total yield.17 We address the problem of carrier photogeneration in a solution-processed BHJ comprised of the molecular donor pDTS(PTTh2)212 and the common electron acceptor PC70BM (see Figure 1a for molecular structures). We use transient absorption spectroscopy to monitor the dynamics of charge carriers in p-DTS(PTTh2)2 that are generated after the absorption of a 400 nm pulse of light. In order to establish a relationship between the charge generation dynamics of this molecular BHJ with the more well-known polymer systems, we also study the PCDTBT:PC70BM BHJ material18 (molecular structure also shown in Figure 1). The second part of this work focuses on the yet unreported photodynamics of p-DTS(PTTh2)2 in order to clarify several questions that are important to understanding the charge generation mechanism of a BHJ solar cell. Of particular importance is the nature of the primary photoexcitations and their relationship to mobile carriers. A model for charge separation uses the language and concepts derived from an understanding of these elementary processes, and subtle clarifications of these issues can do much to alleviate points of confusion. The problem of understanding the relationship between elementary photoexcitations and charge carriers can be Received: September 12, 2012

A

dx.doi.org/10.1021/ja308949m | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

Article

Figure 1. Compounds used in this study.

voltaic systems. Particular attention is paid to the separation of CT excitons, a problem of widespread interest. In addition, the importance of ultrafast carrier generation processes is highlighted.

approached by investigating the photoconductive response of the pure material. The commonly held viewpoint is that photoexciting an organic solid results in highly localized and strongly bound Frenkel excitons. The dissociation of Frenkel excitons is typically discussed in terms of the Onsager−Braun model, originally devised to describe photoionization of gaseous molecules19 and later extended to describe exciton dissociation in the presence of an electric field.20 In this model, electrons and holes are thought to be highly localized (r = ∼1 nm), allowing one to neglect electronic interactions between neighboring molecules. The high degree of localization implies that the Coulomb binding energy between electrons and holes contributes significantly to stabilize bound excitons relative to separated and uncorrelated electron−hole pairs. This energy, called the exciton binding energy, is estimated to be between 1 and 0.1 eV. In the large binding energy limit, one could rightly call the material an insulator, as it would possess negligible photoconductive response within the UV−visible−IR regions of the spectrum. In the case where the photon energy was sufficiently large to overcome the exciton binding energy, carriers could be produced efficiently. Therefore, one of the main indicators for strong exciton binding energies is a photoconductive response that changes as a function of wavelength, conspicuously increasing at some threshold photon energy. The existence of strongly bound Frenkel excitons creates a challenge of understanding charge separation in any context, even in the case of bulk heterojunctions, where a type 2 interface provides the driving force for charge separation. The results of the first two sections are discussed in terms of the general problem of charge photogeneration in organic photo-

■

EXPERIMENTAL SECTION

Films used in transient absorption and fluorescence experiments were spin-cast onto 0.5 in. sapphire disks that were cleaned with piranha solutions for at least 24 h. Sapphire disks were rinsed with deionized water and 2-propanol prior to drying with a dry nitrogen stream. Photoconductivity and absorptance measurements were conducted on glass substrates. All films were spin-cast and annealed inside of a glovebox. Samples were held under high vacuum (p ≈ 10−6 Torr) during the transient absorption and photoconductivity measurements. Pure films of p-DTS(PTTh2)2 were spin-cast from chlorobenzene solutions at a concentration of 40 mg/mL using a spin speed of 1500 rpm. Films were annealed at 60 °C to drive out residual solvent. Bulk heterojunction films of p-DTS(PTTh2)2:PC70BM (7:3 w/w) were prepared in a manner analogous to those used in device fabrication.21 Films were spin-cast from chlorobenzene solutions (40 mg/mL) that contained a small amount of solvent additive (0.25% 1,8-diiodooctane v/v). Films were cast at 1700 rpm and annealed for 10 min at 70 °C. Bulk heterojunction films of PCDTBT:PC70BM (1:4 w/w) were prepared as described in detail in previous reports18 from a solution of 1,2-dichlorobenzene/chlorobenzene (3:1 v/v). Films were spin-cast at 4000 rpm and annealed at 60 °C to drive out residual solvent. Absorptance was determined from the measured transmittance and reflectance of thin films deposited on low-sodium glass substrates using a Perkin-Elmer Lambda 780 NIR−UV−vis spectrometer coupled to an integrating sphere attachment. Photoconductivity measurements were performed in a lateral direction on thin-film samples with gold electrodes using a Keithley 487 picoammeter in conjunction with a Stanford Instruments SR830 lock-in amplifier. A mechanically chopped, monochromated tungsten lamp served as the B

dx.doi.org/10.1021/ja308949m | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

Article

Figure 2. Transient absorption spectra. (a) Spectra collected from a p-DTS(PTTh2)2:PC70BM BHJ material. Pump intensity was 100 μJ cm−2. (b) Spectra collected from a PCDTBT:PC70BM BHJ material. Pump intensity was 200 μJ cm−2. (c) Spectra collected from a p-DTS(PTTh2)2:PC70BM BHJ material at 2 ps (red lines, right axis) and 1 ms (blue dots, left axis); range of axes was adjusted such that they share the same zero and the signal intensities matched in the near-IR. Part d is the same as part c but with a PCDTBT:PC70BM BHJ material.

■

excitation source in the photoconductivity experiment. Photocurrent was referenced to a calibrated silicon photodiode. Fluorescence measurements were conducted using a custom-built fluorimeter. Excitation was accomplished with the 457 nm line of an Ar+ laser (Spectraphysics Beamlok 2065). The fluorescence was collected by a system of lenses and dispersed by a Cherny−Turner monochromator (Acton SP-500). The spectra were recorded using a spectroscopic CCD camera with a Si sensor (Princeton Instruments PIXIS:400). The emission spectra were corrected for the instrument’s spectral response inhomogeneity by taking a spectrum of a calibrated tungsten lamp (Ocean Optics LS-1) and determining the necessary correction factors. Transient absorption measurements were conducted with a pulsed laser system at a repetition rate of 1 kHz. The laser consists of a titanium sapphire oscillator (Spectra Physics Tsunami) that is pumped with a Nd:VO4 laser (Spectra Physics Millenia). The pulses are fed into a regenerative amplifier (Spectra Physics Spitfire) that is pumped with a high-power Nd:YLF laser (Spectra Physics Empower); 790 nm pulses were generated with a pulse width of 100 fs. The pulses were split into pump and probe paths. The pump pulse was frequencydoubled to 395 nm and focused onto the sample with a beam diameter of 1 mm and pulse energies of 0.3−120 μJ/cm2. The pump pulse was put through a delay stage to achieve time resolution. The probe pulse was focused into a 1 mm sapphire disk in order to generate the white light continuum used to measure visible and near-IR spectra. The probe pulse was split before reaching the sample to provide a reference path to aid in the correction of intensity fluctuations. The subtraction was aided by careful collimation of the white light probe. In addition, synchronous chopping of the probe enabled the subtraction of an accurate dark count reading, which tends to drift over time. All spectra were manually corrected for the temporal chirp present in the white light continuum. The polarization angle between pump and probe beams was 54° ± 1° unless otherwise noted. Lastly, spectra were collected with a silicon CCD camera that was calibrated using a series of narrow band-pass filters.

RESULTS Spectral Features of p-DTS(PTTh2)2:PC70BM and PCDTBT:PC70BM BHJ Materials. In order to monitor charge photogeneration and recombination dynamics, the spectral signature of charge carriers must first be unambiguously identified. To do this, we compared transient absorption spectra of both p-DTS(PTTh2)2:PC70BM and PCDTBT:PC70BM BHJ’s at 2 ps and 1 ms (Figure 2c,d). The spectral match in the region 500−800 nm is poor; however the agreement in the near-IR (850−1000 nm) is significantly better. The 1 ms spectra are free of excitons and are dominated by the charge carriers, which would contribute to photocurrent in a solar cell or photoconductor. Thus, the near-IR photoinduced absorption is that of charge carriers. Because the 2 ps spectra have nearly identical line shapes in the near-IR (850−1000 nm) as the 1 ms spectra, and the near-IR line shape is not time-dependent (see Figure 2a,b), the near IR photoinduced absorption can be integrated to provide a probe of the charge carrier population. This assignment agrees with previous reports on a molecule of similar structure to pDTS(PTTh2)2,22 and previous reports on PCDTBT.17,23 The photobleaching signal exhibits a red-shift as a function of time, possibly arising from energy migration,17 although thermal effects24 and acoustic phonons22 might also contribute. Dynamics of p-DTS(PTTh2)2:PC70BM and PCDTBT:PC70BM BHJ Materials. The extracted population dynamics of photoinduced holes was measured at different pump fluences and is shown in Figure 3. Each trace was normalized to its value at 100 fs and its value at negative time scales was set to zero (in effect, subtracting the 1 ms signal). The short time scale behavior found in Figure 3a indicates that a large component of the charge transfer in p-DTSC

dx.doi.org/10.1021/ja308949m | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

Article

both materials there are two easily distinguishable components to the photoinduced charge generation, an ultrafast component, the intensity of which is linear in pump power, and a slower rising component, which is observed at lower pump fluence. Two Pathways for Exciton Splitting in p-DTS(PTTh2)2:PC70BM and PCDTBT:PC70BM BHJ Materials. The different power dependence displayed by the ultrafast charge generation process compared to the slowly rising component clearly indicates that two qualitatively different mechanisms are operating. The slow-rising component, which is strongly affected by the pump intensity, is due to exciton diffusion. As stated previously, high excitation densities can give rise to nonlinear recombination processes, such as exciton− exciton annihilation and charge−exciton recombination.27−29 These processes quench diffusing excitons before they can reach a charge-separating heterojunction interface. Thus, exciton diffusion dynamics will only be observed at low excitation densities, as is the case reported here. It is not immediately obvious, however, whether the excitons are diffusing from fullerene domains or from domains of the electron donor. Because the 400 nm pump pulse excites the fullerene as well as the electron donor, it is reasonable to assume that some part of the diffusive component arises from the fullerene. To address this question, we first observe that the dynamics of exciton diffusion is faster in p-DTS(PTTh2)2:PC70BM than in PCDTBT:PC70BM. This immediately implies that exciton diffusion within the donor plays an important role in the observed dynamics. This observation is confirmed by looking at the dynamics of the photobleach signal. If holes are being transferred into the donor from the fullerene, the photobleach will increase in intensity with time. Conversely, if hole transfer does not occur, and most charges are the result of electron transfer to the fullerene, the intensity of the bleaching signal should be conserved; the strength of the photobleach should not be sensitive to whether it is being caused by an exciton or a hole. In the case of pDTS(PTTh2)2:PC70BM, no such increase is seen. However, some increase (approximately 30%) in the photobleach is observed in PCDTBT:PC70BM heterojunctions, indicating a significant amount of hole transfer to the donor following exciton diffusion in the fullerene. This observation is consistent w it h t he a m ou n t o f f u l l e r en e i n each sampl e. PCDTBT:PC70BM heterojunctions are comprised of 80% fullerene, while p-DTS(PTTh2)2:PC70BM heterojunctions contain 30% fullerene by weight. In summary, the slower rise times of the mobile carrier signal are related to exciton dynamics in both components, but the observed dynamics in pDTS(PTTh2)2:PC70BM BHJ’s is primarily from excitons in pDTS(PTTh2)2. The short time scale (10−20 nm), as required by position−momentum uncertainty. In other words, the initial photoexcitation is delocalized but must undergo rapid localization, or wave function collapse, on the order of the decoherence time scale. If there is wave function amplitude at or near an interface, charge transfer can occur. This process is an important part of the carrier generation process in p-DTS(PTTh2)2:PC70BM (and all BHJ systems studied), providing an explanation of the majority of the most rapidly generated carriers. The remaining ∼30% of photogenerated carriers were created by exciton diffusion to a charge-separating interface at times within 1−500 ps. Strong geminate recombination through bound CT exciton states was not observed on time scales less than 1.5 ns. To explain this observation, we noted the possibility that the CT exciton may not be a true bound state in every circumstance, but a resonance state comprised of mobile electrons and holes in their respective domains.

Figure 10. Schematic illustrations relevant to charge transfer processes. (a) Band diagram of the LUMO region of a heterojunction interface, where the CT exciton is a bound state. Lines indicate three methods of charge generation outlined above. (b) Identical to part a except that the CT exciton is not a bound state, but a resonance is comprised of mobile electrons in the acceptor domain and mobile holes into the donor domain.

■

10a,b represent the electronic states in the LUMO region of the energetic landscape. The binding energy of the CT exciton relative to electronic states in the acceptor (ECT) is marked by a dash at the interface. Similar considerations apply to the hole. We note that Figure 10 is a schematic; quantitative arguments require a two-particle diagram. In Figure 10a we consider the situation when the energy of the CT exciton is below the lowest energy states in the acceptor domain. Such a bound state can decay to the ground state by radiative emission, as has been reported in the literature.56−59 In Figure 10b, we consider the situation when the energy of the CT exciton is greater than the lowest energy states in the acceptor domain. In this case, the CT exciton is not a bound state, but rather, it is a resonance that is comprised of mobile electrons in the acceptor domain and mobile holes into the

AUTHOR INFORMATION

Corresponding Author

[email protected] Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Support for these ultrafast studies was provided by the Center for Energy Efficient Materials, an Energy Frontier Research Center funded by the Office of Basic Energy Sciences of the US Department of Energy (DE-DC0001009). Support for the DFT calculations carried out by R.C.B. was obtained from the National Science Foundation (DMR-1035480). J.J. would like to acknowledge the Australian Solar Institute USASEC research exchange program and the Fulbright Postdoctoral Fellowship. I

dx.doi.org/10.1021/ja308949m | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

Article

(31) Brabec, C. J.; Zerza, G.; Cerullo, G.; De Silvestri, S.; Luzzati, S.; Hummelen, J. C.; Sariciftci, S. Chem. Phys. Lett. 2001, 340, 232. (32) Grancini, G.; Polli, D.; Fazzi, D.; Cabanilas-Gonzalez, J.; Cerullo, G.; Lanzani, G. J. Phys. Chem. Lett. 2011, 2, 1099. (33) Guo, J. M.; Ohkita, H.; Benten, H.; Ito, S. J. Am. Chem. Soc. 2010, 132, 6154. (34) Etzold, F.; Howard, I. A.; Forler, N.; Cho, D. M.; Meister, M.; Mangold, H.; Shu, J.; Hansen, M. R.; Muellen, K.; Laquai, F. J. Am. Chem. Soc. 2012, 134, 10569. (35) Mayer, A. C.; Toney, M. F.; Scully, S. R.; Rivnay, J.; Brabec, C. J.; Scharber, M.; Koppe, M.; Heeney, M.; McCulloch, I.; McGehee, M. D. Adv. Funct. Mater. 2009, 19, 1173. (36) Collins, B. A.; Gann, E.; Guignard, L.; He, X.; McNeill, C. R.; Ade, H. J. Phys. Chem. Lett. 2010, 1, 3160. (37) Miller, N. C.; Gysel, R.; Miller, C. E.; Verploegen, E.; Beiley, Z.; Heeney, M.; McCulloch, I.; Bao, Z. N.; Toney, M. F.; McGehee, M. D. J. Polym. Sci., Part B 2011, 49, 499. (38) Treat, N. D.; Brady, M. A.; Smith, G.; Toney, M. F.; Kramer, E. J.; Hawker, C. J.; Chabinyc, M. L. Adv. Energy Mater. 2011, 1, 82. (39) Liu, F.; Gu, Y.; Jung, J. W.; Jo, W. H.; Russell, T. P. J. Polym. Sci., Part B 2012, 50, 1018. (40) Howard, I. A.; Mauer, R.; Meister, M.; Laquai, F. J. Am. Chem. Soc. 2010, 132, 14866. (41) Smith, M. B.; Michl, J. Chem. Rev. 2010, 110, 6891. (42) Rao, A.; Wilson, M. W. B.; Hodgkiss, J. M.; Albert-Seifried, S.; Bassler, H.; Friend, R. H. J. Am. Chem. Soc. 2010, 132, 12698. (43) Chan, W.-L.; Ligges, M.; Jailaubekov, A.; Kaake, L.; Miaja-Avila, L.; Zhu, X.-Y. Science 2011, 334, 1541. (44) Burdett, J. J.; Bardeen, C. J. J. Am. Chem. Soc. 2012, 134, 8597. (45) Roberts, S. T.; McAnally, R. E.; Mastron, J. N.; Webber, D. H.; Whited, M. T.; Brutchey, R. L.; Thompson, M. E.; Bradforth, S. E. J. Am. Chem. Soc. 2012, 134, 6388. (46) Yang, X. J.; Dykstra, T. E.; Scholes, G. D. Phys. Rev. B 2005, 71, 045203. (47) Hwang, I.; Scholes, G. D. Chem. Mater. 2011, 23, 610. (48) Fluegel, B.; Peyghambarian, N.; Olbright, G.; Lindberg, M.; Koch, S. W.; Joffre, M.; Hulin, D.; Migus, A.; Antonetti, A. Phys. Rev. Lett. 1987, 59, 2588. (49) Jasieniak, J. J.; Hsu, B. B. Y.; Takacs, C. J.; Welch, G. C.; Bazan, G. C.; Moses, D.; Heeger, A. J. ACS Nano 2012, 6, 8735. (50) Kaake, L. G.; Barbara, P. F.; Zhu, X. Y. J. Phys. Chem. Lett. 2010, 1, 628. (51) Heeger, A. J.; Kivelson, S.; Schrieffer, J. R.; Su, W. P. Rev. Mod. Phys. 1988, 60, 781. (52) Tao, S.; Matsuzaki, H.; Uemura, H.; Yada, H.; Uemura, T.; Takeya, J.; Hasegawa, T.; Okamoto, H. Phys. Rev. B 2011, 83, 075204. (53) Arias, A. C.; MacKenzie, J. D.; McCulloch, I.; Rivnay, J.; Salleo, A. Chem. Rev. 2010, 110, 3. (54) Yamagata, H.; Norton, J.; Hontz, E.; Olivier, Y.; Beljonne, D.; Bredas, J. L.; Silbey, R. J.; Spano, F. C. J. Chem. Phys. 2011, 134, 204703. (55) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Springer: New York, 2006. (56) Loi, M. A.; Toffanin, S.; Muccini, M.; Forster, M.; Scherf, U.; Scharber, M. Adv. Funct. Mater. 2007, 17, 2111. (57) Morteani, A. C.; Sreearunothai, P.; Herz, L. M.; Friend, R. H.; Silva, C. Phys. Rev. Lett. 2004, 92, 247402. (58) Huang, Y. S.; Westenhoff, S.; Avilov, I.; Sreearunothai, P.; Hodgkiss, J. M.; Deleener, C.; Friend, R. H.; Beljonne, D. Nat. Mater. 2008, 7, 483. (59) Tvingstedt, K.; Vandewal, K.; Zhang, F. L.; Inganas, O. J. Phys. Chem. C 2010, 114, 21824. (60) Kniepert, J.; Schubert, M.; Blakesley, J. C.; Neher, D. J. Phys. Chem. Lett. 2011, 2, 700. (61) Cowan, S. R.; Roy, A.; Heeger, A. J. Phys. Rev. B 2010, 82, 245207. (62) Street, R. A.; Cowan, S.; Heeger, A. J. Phys. Rev. B 2010, 82, 121301.

We thank Dr. Yanming Sun and Dr. Wei Lin Leong for help with film preparation [with support from AFOSR (FA9550-111-0063)], and Dr. Alexander Mikhailovsky for collecting the fluorescence spectra. We thank Prof. David Awschalom and Prof. Daniel Hone for important discussions.

■

REFERENCES

(1) He, Z.; Zhong, C.; Huang, X.; Wong, W.-Y.; Wu, H.; Chen, L.; Su, S.; Cao, Y. Adv. Mater. 2011, 23, 4636. (2) Dou, L.; You, J.; Yang, J.; Chen, C.-C.; He, Y.; Murase, S.; Moriarty, T.; Emery, K.; Li, G.; Yang, Y. Nat. Photonics 2012, 6, 180. (3) Small, C. E.; Chen, S.; Subbiah, J.; Amb, C. M.; Tsang, S.-W.; Lai, T.-H.; Reynolds, J. R.; So, F. Nat. Photonics 2012, 6, 115. (4) van der Poll, T. S.; Love, J. A.; Nguyen, T.-Q.; Bazan, G. C. Adv. Mater. 2012, 24, 3646. (5) Service, R. F. Science 2011, 332, 293. (6) Bredas, J. L.; Norton, J. E.; Cornil, J.; Coropceanu, V. Acc. Chem. Res. 2009, 42, 1691. (7) Clarke, T. M.; Durrant, J. R. Chem. Rev. 2010, 110, 6736. (8) Howard, I. A.; Laquai, F. Macromol. Chem. Phys. 2010, 211, 2063. (9) Roncali, J. Acc. Chem. Res. 2009, 42, 1719. (10) Li, Y. W.; Guo, Q.; Li, Z. F.; Pei, J. N.; Tian, W. J. Energy Environ. Sci. 2010, 3, 1427. (11) Welch, G. C.; Perez, L. A.; Hoven, C. V.; Zhang, Y.; Dang, X. D.; Sharenko, A.; Toney, M. F.; Kramer, E. J.; Nguyen, T. Q.; Bazan, G. C. J. Mater. Chem. 2011, 21, 12700. (12) Sun, Y.; Welch, G. C.; Leong, W. L.; Takacs, C. J.; Bazan, G. C.; Heeger, A. J. Nat. Mater. 2012, 11, 44. (13) Lee, J.; Vandewal, K.; Yost, S. R.; Bahlke, M. E.; Goris, L.; Baldo, M. A.; Manca, J. V.; Van Voorhis, T. J. Am. Chem. Soc. 2010, 132, 11878. (14) Bakulin, A. A.; Rao, A.; Pavelyev, V. G.; van Loosdrecht, P. H. M.; Pshenichnikov, M. S.; Niedzialek, D.; Cornil, J.; Beljonne, D.; Friend, R. H. Science 2012, 335, 1340. (15) Muntwiler, M.; Yang, Q.; Tisdale, W. A.; Zhu, X. Y. Phys. Rev. Lett. 2008, 101, 196403. (16) Pensack, R. D.; Asbury, J. B. J. Phys. Chem. Lett. 2010, 1, 2255. (17) Etzold, F.; Howard, I. A.; Mauer, R.; Meister, M.; Kim, T.-D.; Lee, K.-S.; Baek, N. S.; Laquai, F. J. Am. Chem. Soc. 2011, 133, 9469. (18) Park, S. H.; Roy, A.; Beaupre, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Nat. Photonics 2009, 3, 297. (19) Wang, G. P.; Zhang, L.; Zhang, J. J. Chem. Soc. Rev. 2012, 41, 797. (20) Braun, C. L. J. Chem. Phys. 1984, 80, 4157. (21) Sun, Y. M.; Welch, G. C.; Leong, W. L.; Takacs, C. J.; Bazan, G. C.; Heeger, A. J. Nat. Mater. 2012, 11, 44. (22) Kaake, L. G.; Welch, G. C.; Moses, D.; Bazan, G. C.; Heeger, A. J. J. Phys. Chem. Lett. 2012, 3, 1253. (23) Tong, M. H.; Coates, N. E.; Moses, D.; Heeger, A. J.; Beaupre, S.; Leclerc, M. Phys. Rev. B 2010, 81, 125210. (24) Albert-Seifried, S.; Friend, R. H. Appl. Phys. Lett. 2011, 98, 223304. (25) Miranda, P. B.; Moses, D.; Heeger, A. J. Phys. Rev. B 2001, 64, 081201. (26) Su, W. P.; Schrieffer, J. R. Proc. Natl. Acad. Sci. U. S. A.-Phys. Sci. 1980, 77, 5626. (27) Kepler, R. G.; Valencia, V. S.; Jacobs, S. J.; McNamara, J. J. Synth. Met. 1996, 78, 227. (28) Howard, I. A.; Hodgkiss, J. M.; Zhang, X.; Kirov, K. R.; Bronstein, H. A.; Williams, C. K.; Friend, R. H.; Westenhoff, S.; Greenham, N. C. J. Am. Chem. Soc. 2010, 132, 328. (29) Hodgkiss, J. M.; Albert-Seifried, S.; Rao, A.; Barker, A. J.; Campbell, A. R.; Marsh, R. A.; Friend, R. H. Adv. Funct. Mater. 2012, 22, 1567. (30) Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Science 1992, 258, 1474. J

dx.doi.org/10.1021/ja308949m | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

Article

(63) Albrecht, S.; Schindler, W.; Kurpiers, J.; Kniepert, J.; Blakesley, J. C.; Dumsch, I.; Allard, S.; Fostiropoulos, K.; Scherf, U.; Neher, D. J. Phys. Chem. Lett. 2012, 3, 640.

K

dx.doi.org/10.1021/ja308949m | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX